I"ve thought of that idea too. I've posted new comments on old posts asking the author to leave a comment so that I can upvote them. I think the main issues is how terrible the user-experience of Steemit.com is (which unfortuantely is still the, by far, most used front-end for the Steem blockchain). Since most ppl use it, there's not real notifications of new comments or mentions on a post and it can easily get lost. If we had private messages, it could also be a game changer.
It's a weak point for Steem and I wonder if Dan will change/fix it when he builds a Steem competitor on EOS. I'm looking fwd to it to say the least.
Upvoting the comment would be a nice way for the author to get payed on a valuable article even after the 7 day period will be over but I don't know. It kinda feels odd. I hope there will be better solution to the problem.